1. Field of the Invention
The present invention relates to a method and apparatus for controlling the differential hydraulic pressure across a variable displacement hydraulic motor which can be used for mechanically actuating a device such as a component on an aircraft.
2. Description of the Prior Art
Aircraft include devices such as flaps, rudders, etc. which must be mechanically actuated during flight to maintain proper control of the aircraft. It is known that these devices can be mechanically actuated by means of a hydraulic motor which is driven by a hydraulic power supply of the aircraft.
It is also known to employ a variable displacement hydraulic motor to mechanically actuate these kinds of devices on an aircraft. The speed-load torque profile of the variable displacement hydraulic motor is such that at high torque and low speed, the motor operates at maximum displacement and near its high speed at minimum displacement. Intermediate the two, the motor operates with a variable displacement, the displacement varying to maintain a constant flow. To maintain a constant flow, it is necessary to maintain a near constant differential pressure across the motor.
A known apparatus for controlling the differential hydraulic pressure across the variable displacement hydraulic motor is shown in FIG. 1 of the drawings. As illustrated therein, the variable displacement hydraulic motor 10 comprises a wobbler 11 which is adjustable for controlling the displacement of the motor. A high-pressure control piston 12 and a low-pressure control piston 13 are operatively connected to the wobbler for adjusting the wobbler to respectively increase and decrease the motor displacement with actuation of the pistons. Pressurized hydraulic fluid is supplied to and returned from the motor 10 by way of the lines b.sub.1 and b.sub.2. A shuttle valve 14 is acted upon by the pressurized hydraulic fluid in both of the lines b.sub.1 and b.sub.2 such that the valve moves to allow communication between the one of the lines b.sub.1 and b.sub.2 having the highest pressure and high pressure line 15 downstream of the shuttle valve 14. The other of the lines b.sub.1 and b.sub.2, having lower pressure is communicated with low pressure line 16 by way of the shuttle valve 14.
The high pressure fluid from high pressure line 15 and the low pressure fluid from low pressure line 16 are communicated to opposite sides of the spool valve of a compensator valve 17. When the differential pressure .DELTA.P across the motor 10 exceeds a predetermined amount, the high pressure in line 15 overcomes the upward force on the spool valve of compensator valve 17 caused by the low pressure from line 16 and the force of a spring in the compensator valve to move the spool valve downward as shown in FIG. 1 thereby communicating the high pressure in line 15 with the high pressure control piston 12. The low pressure in line 16 is communicated with the low-pressure control piston 13 as shown in FIG. 1. Introduction of high pressure fluid to the control piston 12 overcomes the opposing force on the wobbler 11 from the low pressure control piston 13 and of spring 18 to increase the displacement of the motor 10 which, in turn, will decrease the pressure differential .DELTA.P between the lines b.sub.1 and b.sub.2 of the motor 10. In the aforementioned intermediate region of the motor's speed-load torque profile .DELTA.P will decrease until there is a force balance on the spool of the compensator valve 17. This will occur in the vicinity of a predetermined set point. The pressure differential .DELTA.P will exceed the predetermined amount when either a high opposing load or a high aiding load is placed upon the output shaft 19 of the motor 10, resulting in an increase in the motor displacement with both opposing and aiding loads.
The ever increasing performance requirements of advanced aircraft are placing even more demanding peak flow requirements on the hydraulic power supplies. Fully variable displacement motor driven actuation systems offer the potential to significantly reduce the peak flow requirement but do so at the expense of simplicity and cost. There is a need for an improved mechanically controlled variable displacement hydraulic motor driven actuation arrangement which realizes much of the hydraulic flow savings of the fully variable scheme while maintaining the simplicity, low cost and reliability of conventional actuation systems.
Other examples of hydraulic systems employing variable displacement hydraulic motors and control arrangements therefor are shown in U.S. Pat. Nos. 3,465,680; 3,635,021 and 4,478,136.